This application proposes 5 years of support for research examining the causes of comorbidity among substance use disorders (SUD), attention-deficit/hyperactivity disorder (ADHD), and conduct disorder (CD) and the training necessary to conduct this research. SUD, ADHD, and CD are psychiatric disorders that have serious consequences for both the affected individuals and the society. The three disorders occur together more often than the rate expected by chance, and learning the causes of this comorbidity can have a significant impact on the classification, treatment, and research examining the etiology of these disorders. Unfortunately, several family studies testing a hypothesized model for the causes of comorbidity among these disorders used analyses that do not discriminate the hypothesized explanation from any alternative explanation for the cause of comorbidity (Rhee, Hewitt, Corley, & Stallings, 2000). Given the contrasting evidence that the Neale and Kendler (1995) model fitting approach can discriminate classes of alternative comorbidity models reliably (Rhee, Hewitt, Lessem, Stallings, & Corley, 2000), the proposed study will use this approach to examine the comorbidity among SUD, ADHD, and CD. The proposed study will be the first study to use a valid analytic approach to test a wide range of alternative explanations for comorbidity among these disorders using family data. Data already being collected as part of an ongoing Drug Research Center (DRC; DA-11015) will be examined. The DRC samples are a unique combination of a clinically ascertained sample and community family samples (nuclear families, adoptive families, and twins) assessed by a common core of measures. In addition, several extensions of the Neale and Kendler comorbidity models (e.g., multivariate models, multiple thresholds, examination of covariates) will be developed. The candidate will gain the training needed to carry out the proposed study through coursework, seminars, consultations with mentors, workshops, and conferences. The training will take place at the Institute for Behavioral Genetics, a unique environment where the candidate can have regular interactions with several experts in behavior genetics and substance use disorders. Completing this project will assist the candidate in meeting her short-term goal of developing valid methods to examine comorbidity and applying these methods to the causes of comorbidity among ADHD, CD, and SUD. The award will assist the candidate in meeting her long-term career goals of establishing an independent research career in the behavior genetics of disruptive disorders and extending her research to examining specific genetic and environmental influences on these disorders.